1. Field of the Invention
Embodiments of the invention relate to the field of ion implantation. More particularly, the present invention relates to end terminations for electrodes used in ion implant tools to control unwanted beam aberrations.
2. Discussion of Related Art
Ion implantation is a process used to dope ions into a work piece. One type of ion implantation is used to implant impurity ions during the manufacture of semiconductor substrates to obtain desired electrical device characteristics. Typically, arsenic or phosphorus may be doped to form n-type regions in the substrate and boron, gallium or indium is doped to create p-type regions in the substrate.
An exemplary high current ion implanter tool 100 is generally shown in FIG. 1 and includes an ion source chamber 102, and a series of beam line components that direct the ion beam onto a wafer or substrate. These components are housed in a vacuum environment and configured to provide ion dose levels with low energy for shallow ion implantation. In particular, implanter 100 includes an ion source chamber 102 to generate ions of a desired species. The chamber has an associated heated filament powered by power supply 101 to ionize feed gas introduced into the chamber 102 to form charged ions and electrons (plasma). The heating element may be, for example, an indirectly heated cathode. Different feed gases are supplied to the source chamber to generate ions having particular dopant characteristics. The ions are extracted from source chamber 102 via a standard three (3) extraction electrode configuration used to create a desired electric field to focus ion beam 95 extracted from source chamber 102. Beam 95 passes through a mass analyzer chamber 106 having a magnet which functions to pass only ions having the desired charge-to-mass ratio to a resolving aperture. In particular, the analyzer magnet includes a curved path where beam 95 is exposed to the applied magnetic field which causes ions having the undesired charge-to-mass ratio to be deflected away from the beam path. Deceleration stage 108 (also referred to as a deceleration lens) includes a plurality of electrodes (e.g. three) with a defined aperture and is configured to output the ion beam 95. A magnet analyzer 110 is positioned downstream of deceleration stage 108 and is configured to deflect the ion beam 95 into a ribbon beam having parallel trajectories. A magnetic field may be used to adjust the deflection of the ions via a magnet coil. The ribbon beam is targeted toward a work piece which is attached to a support or platen 114. An additional deceleration stage 112 may also be utilized which is disposed between collimator magnet chamber 110 and support 114. Deceleration stage 112 (also referred to as a deceleration lens) may also include a plurality of electrodes (e.g. three) to implant the ions into the substrate at a desired energy level. Because the ions lose energy when they collide with electrons and nuclei in the substrate, they come to rest at a desired depth within the substrate based on the acceleration energy.
The ribbon beam is typically orthogonally directed to the work piece supported by platen 114 to implant the ions into the crystal lattice of the work piece. The depth of ion implantation is based on the ion implant energy and ion mass. Smaller electronic device sizes require high beam current densities implanted at low energy levels (for example ≦2 keV). Typically, low energy ion beams diverge as they travel through an ion implanter because of beam “blow-up” which is due to the effect of the space charge. This effect is caused by the mutual repulsion of the positively charged ions in the beam causing the beam to diverge and grow in cross section. Thus, it has become typical to extract the beam from a plasma and analyze the desired ions at a relatively high energy and then only decelerate the beam to the final energy near the end of the beamline. This is accomplished by a deceleration lens. By applying different combinations of voltage potentials to the plurality of electrodes, the deceleration lens (108, 112) manipulates ion energies associated with the ion beam 95 and cause the ion beam to hit the work piece at a desired energy level to control implantation. In particular, the ions in an ion beam may be decelerated using lens (108, 112) without changing direction of the beam as it passes through the respective lens. However, the voltage potentials applied to the electrodes to decelerate the ions also produce aberrations in the equipotentials at the edges of the electrodes which distort or spread the beam shape. This problem typically arises in a ribbon beam high current tool as described above, but also occurs in a scanned beam tool that can be aimed at high current, medium current or high energy applications. In these cases the wide dimension of the effective beam is created by a narrow pencil beam that sweeps backwards and forwards at high frequency.
In order to correct for or minimize these aberrations, prior attempts have been made to change the geometry of the electrodes. For example, electrodes in a deceleration lens have been bent or angled at the edges to focus the beam and counteract the spreading effect. This attempt has achieved limited success. Alternatively, the electrodes have been configured to extend wider across the beam such that these aberrations are present far outside the beam path. This can be successful, however, when the widths of the electrodes are restrained by the mechanical structure of the vacuum chamber within the implanter. Thus, there is a need for a lens having a plurality of electrodes that terminates an associated electrostatic field of an electrode across the beam path without introducing aberrations that distort beam optics as the beam travels through the lens.